Cathode composite material and lithium ion battery using the same

ABSTRACT

A cathode composite material includes a cathode active material and a coating layer coated on a surface of the cathode active material. The cathode active material includes a lithium cobalt oxide. The coating layer includes a lithium metal oxide having a crystal structure belonging to C2/c space group of the monoclinic crystal system. The present disclosure also relates to a lithium ion battery including the cathode composite material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 from China Patent Application No. 201210373324.0, filed on Sep. 27, 2012 in the China Intellectual Property Office, the content of which is hereby incorporated by reference. This application is related to commonly-assigned applications entitled, “CATHODE COMPOSITE MATERIAL, METHOD FOR MAKING THE SAME, AND LITHIUM ION BATTERY USING THE SAME”, filed Nov. 30, 2012 U.S. Ser. No. 13/690,349; “CATHODE COMPOSITE MATERIAL, METHOD FOR MAKING THE SAME, AND LITHIUM ION BATTERY USING THE SAME”, filed Nov. 30, 2012 U.S. Ser. No. 13/690,334; “CATHODE COMPOSITE MATERIAL AND LITHIUM ION BATTERY USING THE SAME”, filed Nov. 30, 2012 U.S. Ser. No. 13/690,410; “CATHODE COMPOSITE MATERIAL AND LITHIUM ION BATTERY USING THE SAME”, filed Nov. 30, 2012 U.S. Ser. No. 13/691,518; “CATHODE COMPOSITE MATERIAL AND LITHIUM ION BATTERY USING THE SAME”, filed Nov. 30, 2012 U.S. Ser. No. 13/691,521; “CATHODE COMPOSITE MATERIAL AND LITHIUM ION BATTERY USING THE SAME”, filed Nov. 30, 2012 U.S. Ser. No. 13/691,528.

BACKGROUND

1. Technical Field

The present disclosure relates to cathode active materials used in rechargeable lithium ion batteries, and particularly to a cathode composite material used in a rechargeable lithium ion battery.

2. Description of Related Art

Lithium ion batteries can experience capacity loss during charging and discharging cycles and a poor cycling life due to the capacity loss. One reason for the capacity loss during the cycling is an un-reversible change of an intrinsic structure of a cathode active material that occurs during cycling. Another reason for the capacity loss during the cycling is a cathode active material loss caused by a reaction between the cathode active material and the organic solvent that occurs when the cathode active material is in a lithium ion deintercalation state and thus has a high oxidability. Therefore, how to improve the stability of the cathode active material during cycling of the battery and decrease unwanted reactions in the battery is crucial in improving the cycling life of the lithium ion battery.

A common method to improve the conductive properties of cathode active materials of lithium batteries is to treat the surface of the materials. For example, compared to untreated LiFePO₄, the carbon coated LiFePO₄ particles have improved conductivity. However, the stability improving problem during the cycling of the lithium ion battery has not been properly solved.

What is needed, therefore, is to provide a cathode composite material having a relatively good cycling stability and a lithium ion battery using the same.

BRIEF DESCRIPTION OF THE DRAWING

Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, the emphasis instead being placed upon clearly illustrating the principles of the present embodiments.

FIG. 1 is a view of an embodiment of a cathode composite material of a lithium ion battery.

FIG. 2 is a view of an embodiment of a method for making the cathode composite material of the lithium ion battery.

FIG. 3 is a view of an embodiment of a method for making a composite precursor in the method for making the cathode composite material of the lithium ion battery.

FIG. 4 shows an X-ray diffraction (XRD) pattern of Co doped Li₂TiO₃ coated LiCoO₂ of Example 1.

FIG. 5 is an enlarged view of a portion of FIG. 4.

FIG. 6 is a graph comparing cycle performance testing results of lithium ion batteries of Example 1 and Comparative Example 1.

FIG. 7 is a graph comparing cycle performance testing results of lithium ion batteries of Example 1 and Comparative Example 1 using different current rates.

FIG. 8 is a cross-sectional view of an embodiment of a lithium ion battery.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “another,” “an,” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one.

Referring to FIG. 1, one embodiment of a cathode composite material 10 of a lithium ion battery includes a cathode active material particle 12 and a coating layer 14 coated on a surface of the cathode active material particle 12.

Cathode Active Material

The cathode active material particle 12 can be any commonly used cathode active material of lithium ion battery. For example, the material of the cathode active material particle 12 can be a lithium transition metal oxide. The transition metal of the lithium transition metal oxide can be at least one of cobalt (Co), nickel (Ni), and manganese (Mn). The lithium transition metal oxide can have a crystal structure of spinel type, layered type, or olivine type. In one embodiment, the lithium transition metal oxide can be a layered type lithium cobalt oxide. The layered type lithium cobalt oxides can have a crystal structure of an α-NaFeO₂ type layered structure.

The layered type lithium cobalt oxide can be represented by a chemical formula of Li_(x)Co_(1-y)L_(y)O₂, wherein 0.1≦x≦1.1, 0≦y<1. In one embodiment, 0.1<y<0.5. L represents at least one of the chemical elements of alkali metal elements, alkaline-earth metal elements, Group-13 elements, Group-14 elements, transition metal elements, and rare-earth elements. In one embodiment, L represents at least one of the chemical elements of Mn, Ni, chromium (Cr), vanadium (V), titanium (Ti), tin (Sn), copper (Cu), aluminum (Al), iron (Fe), boron (B), strontium (Sr), calcium (Ca), gallium (Ga), neodymium (Nd), and magnesium (Mg). In one embodiment, y=0, and the formula is LiCoO₂.

The shape of cathode active material particle 12 is not limited and can be sphere shape, rod shape, needle shape, sheet shape, tube shape, or irregular shape. A cathode electrode of a lithium ion battery can include a plurality of cathode active material particles 12 in a powder form.

The size of a single cathode active material particle 12 can be chosen according to need and can be in a range from about 1 micron to about 500 microns.

Coating Layer

The coating layer 14 can be an in situ formed layer on the surface of the cathode active material particle 12 and can be a continuous material layer having a uniform thickness. The material of the coating layer 14 can be a lithium metal oxide having a crystal structure belonging to C2/c space group of the monoclinic crystal system. A general chemical formula of the lithium metal oxide can be Li₂AO₃, wherein A represents a metal element which has a +4 valence. In one embodiment, the A of the Li₂AO₃ can be selected from at least one of Ti, Sn, Mn, lead (Pb), tellurium (Te), ruthenium (Ru), hafnium (Hf), and zirconium (Zr). For example, the lithium metal oxide can be at least one of Li₂TiO₃, Li₂MnO₃, Li₂SnO₃, Li₂PbO₃, Li₂TeO₃, Li₂RuO₃, Li₂HfO₃, and Li₂ZrO₃.

The lithium metal oxide can be undoped, or doped with doping chemical elements. The transition metal atoms of the cathode active material particle 12 can diffuse into the coating layer 14 to form the doped lithium metal oxide. The lithium metal oxide can also be represented by a general formula of [Li_(1-2a)M_(a)□_(a)][Li_(1/3-2b-c)M_(b)N_(3c)A_(2/3-2c)□_(b)]O₂, wherein M and N represent doping chemical elements, “□” represents an atom vacancy, which occupies a Li site in the [Li][Li_(1/3)A_(2/3)]O₂, 0≦2a<1, 0≦2b+c<1/3, and 0≦2c<2/3. In the general formula (9, atoms of Li_(1-2a)M_(a)□_(a) are located on inter-plane octahedral sites, atoms of Li_(1/3-2b-c)M_(b)N_(3c)A_(2/3-2c)□_(b) are located on in-plane octahedral sites. More specifically, the undoped Li₂AO₃ can also be represented by the chemical formula of [Li]^(α)[Li_(1/3)A_(2/3)]^(β)O₂, which is a transform of the formula Li₂AO₃, wherein [ ]^(α) represents that the atoms in the [ ]^(α) are located on the inter-plane octahedral sites, which are occupied by Li, and [ ]^(β) represents that the atoms in the [ ]^(β) are located on the in-plane octahedral sites, which are occupied by Li and A in a molar ratio of 1/3:2/3. When the ions of the doping element M replace an amount of Li⁻ located on the inter-plane octahedral sites and in-plane octahedral sites, an equal amount of atom vacancies (□) are also generated. The atom vacancies (□) are helpful for Li⁻ moving in the coating layer 14. Further, the ions of the doping element N can replace an amount of Li⁺ and A⁴⁺ of the Li_(1/3)A_(2/3) located on the in-plane octahedral sites.

More specifically, M and N respectively represent at least one of the chemical elements of alkali metal elements, alkaline-earth metal elements, Group-13 elements, Group-14 elements, transition metal elements, and rare-earth elements. In one embodiment, M and N respectively represent at least one of the chemical elements of Co, Mn, Ni, Cr, V, Ti, Sn, Cu, Al, Fe, B, Sr, Ca, Ga, Nd, and Mg.

At least one of M and N can come from the lithium transition metal oxide of the cathode active material particle 12. When the lithium transition metal oxide is the lithium cobalt oxide, at least one of M and N can be Co. For example, when N is Co, the general formula of the material of the coating layer can be [Li][Li_(1/3-c)Co_(3c)A_(2/3-2c)]O₂, and when A is Ti, the general formula of the material of the coating layer can be [Li][Li_(1/3-c)Co_(3c)Ti_(2/3-2c)]O₂, wherein the Co³⁺ can replace an amount of the Li⁺ and Ti⁴⁺ on the in-plane octahedral sites, to make the structure of the Li₂TiO₃ of monoclinic crystal system similar to the layered type LiMO₂. Therefore, the combination between the coating layer 14 and the cathode active material particle 12 having the LiMO₂ type structure can be improved.

A mass percentage of the coating layer 14 to the cathode composite material 10 can be in a range from about 0.05% to about 7%. In one embodiment, the mass percentage of the coating layer 14 is about 5%. A thickness of the coating layer 14 can be in a range from about 2 nanometers (nm) to about 20 nm.

The cathode electrode of the lithium ion battery can include a plurality of cathode active material particles 12. The coating layer 14 can be individually coated on the individual cathode active material particle 12. The cathode electrode of the lithium ion battery can include a plurality of core-shell structures. Each core-shell structure has one cathode active material particle 12 as the core and one coating layer 14 as the shell. In one embodiment, each of the plurality of cathode active material particles 12 in the cathode electrode has the coating layer 14 coated thereon. The coating layer 14 can completely coat an entire outer surface of the individual cathode active material particle 12. The coating layer 14 can have a uniform thickness, and thus the cathode composite material 10 can have a sphere shape, rod shape, needle shape, sheet shape, tube shape, or irregular shape corresponding to the shape of the cathode active material particle 12.

Method for Making the Cathode Active Material

One embodiment of a method for preparing the cathode composite material of the lithium ion battery includes steps of:

-   -   S1, forming a composite precursor including a cathode active         material precursor and a coating layer precursor coated on a         surface of the cathode active material precursor;     -   S2, reacting the composite precursor with a lithium source         chemical compound, to lithiate both the cathode active material         precursor and the coating layer precursor in the composite         precursor.

The cathode active material precursor is capable of having a chemical reaction with the lithium source chemical compound to produce the cathode active material. In one embodiment, the cathode active material precursor is capable of having a chemical reaction with the lithium source chemical compound to produce the layered type lithium cobalt oxide. The cathode active material precursor is used as a cobalt source. The material of the cathode active material precursor can be an oxygen compound of the cobalt. The oxygen compound can be at least one of hydroxides, oxysalts, and oxides of cobalt. The oxysalt is a salt of an oxyacid. The oxysalt can be at least one of oxalates, carbonates, acetates, and oxyhydroxides (i.e., oxide-hydroxides). For example, the oxysalt of the transition metal can be at least one of cobalt hydroxide (Co(OH)₂), cobalt oxalate (CoC₂O₄.2H₂O), cobalt carbonate (CoCO₃), cobalt acetate(Co(CH₃COO)₂.4H₂O), cobalt oxide-hydroxide (CoOOH). The oxygen compound of the cobalt can be with or without the crystallization water.

The material of the coating layer precursor can be hydroxides or oxides of the metal element which has the +4 valence. For example, the material of the coating layer precursor can be at least one of titanium dioxide (TiO₂), stannic dioxide (SnO₂), manganese dioxide (MnO₂), lead dioxide (PbO₂), tellurium dioxide (TeO₂), ruthenium dioxide (RuO₂), hafnium dioxide (HfO₂), and zirconium dioxide (ZrO₂).

The material of the lithium source chemical compound can be at least one of lithium hydroxide, lithium chloride, lithium sulfate, lithium nitrate, lithium acetate, lithium dihydrogen orthophosphate.

Referring to FIG. 2, the above described method can be used to form the cathode composite material 10. The composite precursor 100 can include the coating layer precursor 140 and the cathode active material precursor 120. The coating layer precursor 140 is coated on the cathode active material precursor 120. The coating layer precursor 140 is capable of having a chemical reaction with the lithium source chemical compound to produce the coating layer 14. That is to say, the coating layer precursor 140 is a chemical compound which can have a chemical reaction with the lithium source chemical compound to produce the lithium metal oxide having a crystal structure belonging to the C2/c space group of the monoclinic crystal system. The cathode active material precursor 120 is capable of having a chemical reaction with the lithium source chemical compound to produce the cathode active material particle 12. The coating layer precursor 140 is a continuous layer shaped structure, which is in-situ formed on the outer surface of the cathode active material precursor 120. The lithium source chemical compound 160 can simultaneously react with the cathode active material precursor 120 and the coating layer precursor 140, to produce the cathode active material particle 12 and the coating layer 14 coated on the cathode active material particle 12. The cathode active material precursor 120 can have a particle shape. In one embodiment, a plurality of cathode active material precursors 120 are provided, and the coating layer precursor 140 is individually coated on the individual cathode active material precursor 120.

In one embodiment, the step Si can further include steps of:

S11, dispersing the plurality of cathode active material precursors in a liquid solvent to form a solid-liquid mixture, and the plurality of cathode active material precursors are insoluble to the liquid solvent;

S12, adding a coating substance in the solid-liquid mixture;

S13, heating the solid-liquid mixture having the coating substance added therein to form the coating substance into the coating layer precursor on the outer surface of the cathode active material precursor, thereby forming the composite precursor.

In one embodiment, when the oxygen compound of the cobalt is at least one of cobalt hydroxides and cobalt oxysalts, or when the oxygen compound of the cobalt carries the crystallization water, the liquid solvent can only be an organic solvent (i.e., without water). The cobalt hydroxides, cobalt oxysalts, or the crystallization water can generate water in the solid-liquid mixture due to the heating step. The water reacts with the coating substance. The organic solvent can be a commonly used solvent such as methanol, ethanol, propanol, isopropanol, ethylene glycol, acetone, dichloroethane, and chloroform.

The coating substance can be a liquid state substance, a liquid solution, or a substance that is soluble to the liquid solvent. The coating substance can include at least one of metal halides and metal esters. In one embodiment, the metal element in the metal halides and metal esters has the +4 valence. More specifically, the coating substance can be at least one of diethyl titanate, tetrabutyl titanate, zirconium n-butoxide, tetrabutyl orthostannate, titanium tetrachloride (TiCl₄), zirconium tetrachloride (ZrCl₄), stannic tetrachloride (SnCl₄), lead tetrachloride (PbCl₄), tellurium tetrachloride (TeCl₄), ruthenium tetrachloride (RuCl₄), hafnium tetrachloride (HfCl₄), manganese tetrachloride (MnCl₄).

In the step S11, the plurality of cathode active material precursor can be uniformly dissolved in the liquid solvent through an ultrasonic vibration or a mechanical stirring method.

In the step S12, a molar ratio of the coating substance to the cathode active material precursor can be in a range from about 0.1:100 to about 20:100. The coating substance can be uniformly mixed with the solid-liquid mixture through an ultrasonic vibration or a mechanical stirring method.

In the step S13, the liquid-solid mixture having the coating substance added therein can be directly heated in an open environment in air or heated by using a hydrothermal synthesis method in an autoclave, which is sealed and has a pressure larger than 1 atm therein during the heating. The heating temperature can be decided according to the pressure and the material of the coating substance. In one embodiment, the heating temperature can be in a range from about 80° C. to about 200° C. The heating time can be decided according to the heating temperature and the amount of the coating substance. In one embodiment, the heating time can be in a range from about 10 minutes to about 20 hours. In the heating step of the step S13, the coating substance can have a hydrolysis reaction to produce the coating layer precursor on the outer surface of the cathode active material precursor.

Referring to FIG. 3, when the cathode active material precursor 120 carries the crystallization water 180, during the heating step of the step S13, the crystallization water can escape from the cathode active material precursor 120 to form a liquid state water. The liquid state water can have a reaction with the coating substance 144 to form the coating layer precursor 140 coated on the surface of the crystallization water removed cathode active material precursor 120′.

By using the liquid solvent without the water, and providing the water only by the crystallization water of the cathode active material precursor, the reaction between the water and the coating substance is easier to be controlled and the morphology of the coating layer precursor can be formed well. Accordingly, the uniformity of the coating layer precursor can be improved.

In another embodiment, the step S1 can include steps of:

S11′, dispersing the plurality of cathode active material precursor in a liquid solvent to form a solid-liquid mixture, and the plurality of cathode active material precursor is insoluble to the liquid solvent.

S12′, adding a coating substance in the solid-liquid mixture;

S13′, reacting the coating substance with water to have a hydrolysis reaction, to produce the coating layer precursor on the outer surface of the cathode active material precursor, thereby forming the composite precursor.

The steps S11′ to S13′ is similar to the steps S11 to S13, except that when the cathode active material precursor 120 itself does not carries the crystallization water, an additional step of reacting the coating substance with an added water can be used to hydrolyses the coating substance. More specifically, in one embodiment, the liquid solvent can be a mixture of the organic solvent and a small amount of water. A volume ratio between the water and the organic solvent can be equal to or smaller than 1:10 (in some embodiments is smaller than 1:50). In another embodiment, when the liquid solvent only includes the organic solvent, the step S13′ can further include a step of adding water into the solid-liquid mixture. The amount of the added water can be decided by the amount of the coating substance. Overall, the solid-liquid mixture can include a small amount of water. The amount of the water can be decided by the amount of the coating substance.

In the step S2, the composite precursor is reacted with the lithium source chemical compound, the cathode active material precursor and the coating layer precursor can both be lithiated. A high-temperature solid-state reaction can be used to react the composite precursor with the lithium source chemical compound. More specifically, the step S2 can include steps of uniformly mixing the composite precursor with the lithium source chemical compound to form a solid-solid mixture and heating the solid-solid mixture, under a solid-state reaction temperature. The composite precursor and the lithium source chemical compound can be mixed through a mechanical stirring method or a ball milling method. The solid-state reaction temperature can be decided according to the material of the composite precursor. In one embodiment, the solid-state reaction temperature can be in a range from about 400° C. to about 900° C., and the heating time can be in a range from about 2 hours to about 12 hours.

In one embodiment, before heating the solid-solid mixture under the solid-state reaction temperature, an additional step of pre-heating the solid-solid mixture under a relatively low temperature can be processed for fully decomposing the composite precursor. The relatively low temperature can be in a range from about 350° C. to about 400° C., and the pre-heating time can be in a range from about 0.5 hours to about 1 hour. After the pre-heating step, the solid-solid mixture can be directly heated to reach the solid-state reaction temperature without a cooling step.

An amount of the lithium source chemical compound can be decided by the amount of the composite precursor. More specifically, the amount of the lithium source chemical compound can be calculated from the lithium amount required for lithiating the cathode active material precursor and the coating layer precursor. Moreover, for the reason that the lithium is apt to be volatilized in a high temperature, the lithium source chemical compound can be excess for the composite precursor. In one embodiment, the stoichiometric ratio between the lithium source chemical compound and the composite precursor can be in a range from about 1:1 to about 1.2:1. During the lithiation of the cathode active material precursor to produce the cathode active material, the lithium source chemical compound also reacts with the coating layer precursor, and directly forms the coating layer on the surface of the formed cathode active material. The coating layer precursor and the cathode active material precursor are simultaneously heated at the solid state reaction temperature, and both form the lithiated substances; thus, there is a strong chemical bonding force between the formed cathode active material and the coating layer, to make the coating layer continuously, tightly and uniformly coated on the surface of the cathode active material.

It is to be understood that the coating substance can include more than one metal elements which having the +4 valence. Thus, the finally formed coating layer 14 can include more than one lithium metal oxide mixed with each other. Moreover, the cathode active material precursor can also include more than one oxygen compound of the transition metals. Thus, the finally formed cathode active material particle 12 can include more than one transition metal elements. Moreover, the cathode active material precursor can also include at least one of the above mentioned doping elements L. Thus, the finally formed cathode active material particle 12 can be a doped lithium transition metal oxide, for example, a doped lithium cobalt oxide.

The cathode composite material of the present disclosure includes the coating layer coated on the cathode active material to avoid a direct contact between the cathode active material and the electrolyte solution. The material of the coating layer can be a lithium metal oxide having a crystal structure belonging to C2/c space group of the monoclinic crystal system. Thus, the lithium metal oxide has a one dimensional lithium ion transferring path along a crystal axis c of the crystal structure of the lithium metal oxide. Therefore, the coating layer can prevent an electron migration between the electrolyte and the cathode active material and allow the lithium ions to pass therethrough. Thus, the side reaction during the charge and discharge of the lithium ion battery between the electrolyte and the cathode active material can be suppressed. Therefore, the cathode composite material has improved chemical and thermal stabilities, even at a relatively high or low charge/discharge voltage, or a high rate.

In an embodiment of the method, a composite precursor having a core-shell structure is previously prepared, and then the core and the shell of the core-shell structure are simultaneously lithiated, to form the cathode composite material and the cathode active material at the same time. The in situ formed coating layer is an integrated and continuous material layer having uniform thickness, not just a plurality of particles loosely stacked on the cathode active material. The coating layer and the cathode active material have a tight combination, which effectively decreases the dislocation and increase the lithium ion mobility in the coating layer. Therefore, the lithium ion battery using the cathode composite material has a relatively good cycle performance.

Lithium Ion Battery

Referring to FIG. 8, one embodiment of a lithium ion battery 1 includes a cathode electrode 2, an anode electrode 3 spaced from the cathode electrode 2, and a non-aqueous electrolyte 8 disposed between the cathode electrode 2 and the anode electrode 3.

The cathode electrode 2 can include a cathode current collector 24 and a cathode material layer 22. The cathode current collector 24 is used for supporting the cathode material layer 22 and conducting current. A shape of the cathode current collector 24 can be a continuous sheet or a network. A material of the cathode current collector 24 can be metal or alloy, such as aluminum, titanium, or stainless steel. The cathode material layer 22 is disposed on at least one surface of the cathode current collector 24. The cathode material layer 22 includes the above described cathode composite material 10 and can further include a conductive agent and a binder. The cathode composite material 10, conductive agent, and the binder can be uniformly mixed with each other.

The anode electrode 3 can include an anode current collector 34 and an anode material layer 32. The anode current collector 34 is used for supporting the anode material layer 32 and conducting current. A shape of the anode current collector 34 can be a continuous sheet or a network. A material of the anode current collector 34 can be metal or alloy, such as copper, nickel, or stainless steel. The anode material layer 32 is disposed on at least one surface of the anode current collector 34. The anode material layer 32 includes an anode active material and can further include a conductive agent and a binder. The anode active material, conductive agent, and the binder can be uniformly mixed with each other. The anode active material particle can be lithium titanate (e.g., Li₄Ti₅O₁₂), graphite, acetylene black, organic cracking carbon, mesocarbon microbeads (MCMB), or any combination thereof.

The anode material layer 32 faces the cathode material layer 22. Both the anode material layer 32 and the cathode material layer 22 are in contact with the electrolyte. The electrolyte 8 can be a solid electrolyte film or a liquid non-aqueous electrolyte solution. The solid electrolyte film can be sandwiched between the anode electrode 3 and the cathode electrode 2. If the lithium ion battery 1 uses the liquid non-aqueous electrolyte solution, the cathode electrode 2 and the anode electrode 3 are both disposed in the liquid non-aqueous electrolyte solution. The lithium ion battery 1 can further include a separator 4 disposed between the anode material layer 32 and the cathode material layer 22, to electrically insulate them from each other. The separator 4 is capable of having the lithium ions pass therethrough. The liquid non-aqueous electrolyte solution includes an organic solvent and a lithium salt dissolved in the organic solvent. The organic solvent can be selected from cyclic carbonates, linear carbonates, cyclic ethers, linear ethers, nitriles, and amides, and can be at least one of ethylene carbonate (EC), propylene carbonate (PC), ethylmethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), butylenes carbonate, vinylene carbonate, methylethyl carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, y-butyrolactone, 1,2dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1,2-dioxane, 2-methyltetrahydrofuran, acetonitrile, and dimethylformamide. The lithium salt may be at least one of LiPF₆, LiBOB, LiBF₄, LiSbF₆, LiAsF₆, LiClO₄, LiCF₃SO₃, Li(CF₃SO₂)₂N, LiC₄F₉SO₃, LiSbF₆, LiAlO₄, LiAlCl₄, LiCl, and LiI. The separator 4 can be a porous membrane. The material of the separator can be polyolefins, such as polypropylene (PP) or polyethylene (PE), or inorganic material such as porous ceramics.

The conductive agents and the binders in the anode material layer 32 and the cathode material layer 22 can be the same. The conductive agent can be a carbonaceous material such as carbon black, acetylene black, conductive polymers, carbon fibers, carbon nanotubes, graphene, and graphite. The binder can be at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and styrene-butadiene rubber (SBR).

The materials of the anode current collector 34, the cathode current collector 24, the conductive agent, the binder, the electrolyte, and the separator 4 are not limited by the above listed materials, and can be selected from other known materials.

The lithium ion battery 1 can further include an exterior encapsulating structure, such as a hard battery case 5 sealed by a sealing member 6, or a soft encapsulating bag, having the cathode electrode 2, the anode electrode 3, the separator 4, the electrolyte 8 located therein. The lithium ion battery 1 can further include a connecting component achieving an electrical connection between the current collector of the lithium ion battery 1 and the external circuit.

EXAMPLES Example 1

In Example 1, 0.25 g Co(OH)₂ is ultrasonically dispersed in 10 mL of ethanol to achieve the solid-liquid mixture. Then, the tetrabutyl titanate is added into the solid-liquid mixture. A molar ratio between the tetrabutyl titanate and the Co(OH)₂ is about 1:10. After adding the tetrabutyl titanate, the solid-liquid mixture is continuously ultrasonically vibrated to uniformly mix the tetrabutyl titanate and the Co(OH)₂ in the ethanol. After that, the solid-liquid mixture is transferred into a dry and clean hydrothermal autoclave and reacted at about 150° C. for about 3 hours. After the hydrothermal synthesis, the amorphous TiO₂ layer can be formed on the surface of the Co(OH)₂, to form the composite precursor. The composite precursor and the LiOH.H₂O are mixed in the stoichiometric ratio and are dried to remove the residual ethanol. Then the mixture of the composite precursor and the LiOH.H₂O are heated to about 800° C. at a temperature increasing rate of about 3° C./min, and is heated at the 800° C. for about 5 hours. The cathode composite material is naturally cooled down to room temperature. The cathode composite material is the core-shell structure having LiCoO₂ as the core and Co doped Li₂TiO₃ as the shell. An XRD analysis is conducted on the cathode composite material. Referring to FIG. 4, the XRD pattern of the cathode composite material shows that a pure phase LiCoO₂ exits in the cathode composite material. Referring to FIG. 5, by enlarging the XRD pattern of FIG. 4, the Li₂TiO₃ peaks can be observed, which shows that the Li₂TiO₃ is formed. Further, the peak of the Li₂TiO₃ near the 23 degrees of the 2 theta value disappears, which means that Co³⁺ is doped in the Li₂TiO₃.

A lithium ion battery is assembled using the cathode composite material of the Example 1 in the cathode electrode.

Comparative Example 1

A lithium ion battery is assembled according to the same conditions as in Example 1, except that the cathode composite material formed in Example 1 is replaced by the bare LiCoO₂ without any coating layer coated thereon.

Electrochemical Experiment of Example 1 and Comparative Example 1

Referring to FIG. 6, the two lithium ion batteries of the Example 1 and Comparative Example 1 are galvanostatically cycled using the 1C current rate. The lithium ion battery of the Example 1 has a higher discharge specific capacity at the first cycle (about 140 mAh/g), and a better capacity retention. After 100 cycles of the charge and discharge, the discharge specific capacity of the lithium ion battery of the Example 1 hardly decreased. On the contrary, the lithium ion battery of the Comparative Example 1 only has the discharge specific capacity of about 128 mAh/g at the first cycle. After 100 cycles of the charge and discharge, the discharge specific capacity of the lithium ion battery of the Comparative Example 1 is below 90 mAh/g.

Referring to FIG. 7, the lithium ion battery of the Comparative Example 1 are galvanostatically cycled using the current rates of 1 C, 2 C, 3 C, 4 C, 5 C, and 10 C. As the current rate increases, the discharge specific capacity of the lithium ion battery of the Comparative Example 1 dramatically decreases. When the current rate gets to 10 C, the discharge capacity of the lithium ion battery of the Comparative Example 1 is only about 110 mAh/g. On the contrary, the lithium ion battery in the Example 1 having the LiCoO₂ coated by the Co doped Li₂TiO₃ coating layer has a better rating performance. The lithium ion battery of the Example 1 is galvanostatically cycled using the current rates of 1 C, 5 C, 10 C, 15 C, 20 C, and 25 C. As the current rate increases, the discharge specific capacity of the lithium ion battery of the Example 1 does not decrease much. When the current rate gets to 10 C, the discharge specific capacity of the lithium ion battery of the Example 1 is still about 137 mAh/g. The discharge specific capacity of the lithium ion battery of the Example 1 at 25 C is also larger than the discharge specific capacity of the lithium ion battery of the Comparative Example 1 at 10 C.

Example 2

In Example 2, the cathode composite material is prepared using the same method as in Example 1, except that the Co(OH)₂ is replaced by CoC₂O₄.xH₂O.

Example 3

In Example 3, the cathode composite material is prepared using the same method as in Example 1, except that the tetrabutyl titanate is replaced by zirconium n-butoxide. After the hydrothermal synthesis, the amorphous ZrO₂ layer can be formed on the surface of the Co(OH)₂, to form the composite precursor. After the lithiating step, the cathode composite material is the core-shell structure having LiCoO₂ as the core and Co doped Li₂ZrO₃ as the shell.

Example 4

In Example 4, the cathode composite material is prepared using the same method as in Example 1, except that the tetrabutyl titanate is replaced by tetrabutyl orthostannate. After the hydrothermal synthesis, the amorphous SnO₂ layer can be formed on the surface of the Co(OH)₂, to form the composite precursor. After the lithiating step, the cathode composite material is the core-shell structure having LiCoO₂ as the core and Co doped Li₂SnO₃ as the shell.

Example 5

In Example 5, the cathode composite material is prepared using the same method as in Example 1, except that the tetrabutyl titanate is replaced by PbCl₄. After the hydrothermal synthesis, the amorphous PbO₂ layer can be formed on the surface of the Co(OH)₂, to form the composite precursor. After the lithiating step, the cathode composite material is the core-shell structure having LiCoO₂ as the core and Co doped Li₂PbO₃ as the shell.

Example 6

In Example 6, the cathode composite material is prepared using the same method as in Example 1, except that the tetrabutyl titanate is replaced by TeCl₄. After the hydrothermal synthesis, the amorphous TeO₂ layer can be formed on the surface of the Co(OH)₂, to form the composite precursor. After the lithiating step, the cathode composite material is the core-shell structure having LiCoO₂ as the core and Co doped Li₂TeO₃ as the shell.

Example 7

In Example 7, the cathode composite material is prepared using the same method as in Example 1, except that the tetrabutyl titanate is replaced by RuCl₄. After the hydrothermal synthesis, the amorphous RuO₂ layer can be formed on the surface of the Co(OH)₂, to form the composite precursor. After the lithiating step, the cathode composite material is the core-shell structure having LiCoO₂ as the core and Co doped Li₂RuO₃ as the shell.

Example 8

In Example 8, the cathode composite material is prepared using the same method as in Example 1, except that the tetrabutyl titanate is replaced by HfCl₄. After the hydrothermal synthesis, the amorphous HfO₂ layer can be formed on the surface of the Co(OH)₂, to form the composite precursor. After the lithiating step, the cathode composite material is the core-shell structure having LiCoO₂ as the core and Co doped Li₂HfO₃ as the shell.

Example 9

In Example 9, the cathode composite material is prepared using the same method as in Example 1, except that the tetrabutyl titanate is replaced by MnCl₄. After the hydrothermal synthesis, the amorphous MnO₂ layer can be formed on the surface of the Co(OH)₂, to form the composite precursor. After the lithiating step, the cathode composite material is the core-shell structure having LiCoO₂ as the core and Co doped Li₂MnO₃ as the shell.

Examples for Doped Lithium Cobalt Oxides

In these examples, the cathode composite material is prepared using the same method as in Example 1, except that an amount of hydroxide of the doping element L is added with the Co(OH)₂. The doping element L is Mn, Ni, Cr, V, I, Sn, Cu, Al, Fe, B, Sr, Ca, Ga, Nd, and Mg each in one example. A molar ratio of the doping element L and the Co(OH)2 is about y: (1-y). After the lithiating step, the cathode composite material is the core-shell structure having LiCo_(1-y)L_(y)O₂ as the core and Co doped Li₂TiO₃ as the shell, wherein 0≦y<1, L is Mn, Ni, Cr, V, I, Sn, Cu, Al, Fe, B, Sr, Ca, Ga, Nd, and Mg each in one example.

Depending on the embodiment, certain steps of methods described may be removed, others may be added, and the sequence of steps may be altered. It is also to be understood that the description and the claims drawn to a method may include some indication in reference to certain steps. However, the indication used is only to be viewed for identification purposes and not as a suggestion as to an order for the steps.

Finally, it is to be understood that the above-described embodiments are intended to illustrate rather than limit the present disclosure. Variations may be made to the embodiments without departing from the spirit of the present disclosure as claimed. Elements associated with any of the above embodiments are envisioned to be associated with any other embodiments. The above-described embodiments illustrate the scope of the present disclosure but do not restrict the scope of the present disclosure. 

What is claimed is:
 1. A cathode composite material comprising a cathode active material and a coating layer coated on a surface of the cathode active material, the cathode active material comprises a lithium cobalt oxide, the coating layer comprises a lithium metal oxide having a crystal structure that belongs to C2/c space group of the monoclinic crystal system, wherein a general formula of the lithium metal oxide is [Li_(1-2a)M_(a)□_(a)][Li_(1/3-2b-c)M_(b)N_(3c)A_(2/3-2c)□_(b)]O₂ wherein A represents a metal element having a +4 valence, M and N respectively represent doping chemical elements, “□” represents an atom vacancy occupying a Li site of [Li][Li_(1/3)A_(2/3)]O₂, 0≦2a<1, 0≦2b+c<1/3, and 0≦2c<2/3.
 2. The cathode composite material of claim 1, wherein A is selected from the group consisting of Ti, Sn, Mn, Pb, Te, Ru, Hf, Zr, and any combination thereof.
 3. The cathode composite material of claim 1, wherein the M and N are respectively selected from the group consisting of alkali metal elements, alkaline-earth metal elements, Group-13 elements, Group-14 elements, transition metal elements, rare-earth elements, and any combination thereof.
 4. The cathode composite material of claim 3, wherein M and N are respectively selected from the group consisting of Co, Mn, Ni, Cr, V, Ti, Sn, Cu, Al, Fe, B, Sr, Ca, Ga, Nd, Mg, and combinations thereof.
 5. The cathode composite material of claim 1, wherein at least one of M and N comes from the cathode active material.
 6. The cathode composite material of claim 1, wherein the general formula of the lithium metal oxide is [Li][Li_(1/3-c)Co_(3c)A_(2/3-2c)]O₂.
 7. The cathode composite material of claim 1, wherein the coating layer is a continuous layer and has a uniform thickness.
 8. The cathode composite material of claim 1, wherein the coating layer is an in situ formed layer on the surface of the cathode active material.
 9. The cathode composite material of claim 1, wherein a mass percentage of the coating layer is in a range from about 0.05% to about 7%.
 10. The cathode composite material of claim 1, wherein a thickness of the coating layer is in a range from about 2 nanometers to about 20 nanometers.
 11. The cathode composite material of claim 1, wherein the cathode active material comprises a plurality of cathode active material particles, and the coating layer is individually coated on the surface of each of the plurality of cathode active material particles.
 12. The cathode composite material of claim 11, wherein the coating layer completely coats an entire outer surface of the each of the plurality of cathode active material particles.
 13. The cathode composite material of claim 1, wherein a size of the cathode active material is in a range from about 1 micron to about 500 microns.
 14. The cathode composite material of claim 13, wherein the cathode composite material has a shape corresponding to the shape of the cathode active material.
 15. The cathode composite material of claim 1, wherein a shape of cathode active material is selected from the group consisting of sphere shape, rod shape, needle shape, sheet shape, tube shape, and irregular shape.
 16. The cathode composite material of claim 1, wherein the cathode active material and the coating layer are combined by a chemical bonding force therebetween.
 17. The cathode composite material of claim 1, wherein the material of the coating layer is selected from the group consisting of Li₂TiO₃, Li₂MnO₃, Li₂SnO₃, Li₂PbO₃, Li₂TeO₃, Li₂RuO₃, Li₂HfO₃, Li₂ZrO₃, and combinations thereof.
 18. A lithium ion battery comprising: an anode electrode; a cathode electrode comprising a cathode composite material comprising a plurality of cathode active material particles and a coating layer coated on a surface of each of the plurality of cathode active material particles, the each of the plurality of cathode active material particles comprises a lithium cobalt oxide, and the coating layer comprises a lithium metal oxide having a crystal structure belonging to C2/c space group of the monoclinic crystal system; and a non-aqueous electrolyte disposed between the cathode electrode and the anode electrode, wherein a general formula of the lithium metal oxide is [Li_(1-2a)M_(a)□_(a)][Li_(1/3-2b-c)M_(b)N_(3c)A_(2/3-2c)□_(b)]O₂, wherein A represents a metal element having a +4 valence, M and N respectively represent doping chemical elements, “□” represents an atom vacancy occupying a Li site of [Li][Li_(1/3)A_(2/3)]O₂, 0≦2a<1, 0≦2b+c<1/3, and 0≦2c<2/3. 